The invention pertains to a hollow walled article manufactured by a rotational molding process, and the process thereof. More particularly, the invention pertains to a rotational molded boat and the process of molding the boat.
Rotational molding is intended primarily for the manufacture of hollow structures from thermoplastic and, to some extent, thermosetting materials.
A solid or liquid polymer is placed in a mold; the mold is heated and then cooled while being rotated about two perpendicular axes simultaneously. During the first portion of the heating stage when molding with powdered material, a porous skin is formed on the mold surface. This gradually melts as the cycle progresses to form a homogeneous layer of uniform thickness adhering to the mold""s surface. When molding a liquid material, it tends to flow and coat the mold""s surface until the gel temperature of the resin is reached, at which time all flow ceases. The shape of the object being molded conforms to the inside surfaces of the mold. The structure is hollow between the molded surfaces.
When all flow ceases, the mold is indexed to a cooling station, where the mold is cooled. It is then positioned in a work zone, where the mold is opened, the finished part removed, and the mold recharged for the next cycle.
Rotational molding of objects, such as boat hulls, is not new. In the case of boats, the process produces a hollow boat hull with an inner and outer walls. The hollow space in between is conveniently filled with foam for purposes of buoyancy and strength. The inner and outer walls are desireably connected for purposes of rigidity of the structure. This is usually accomplished along one or more ribs that run lengthwise of the hull. The rib is formed by closing the mold during the rotational molding process in order to bring together and fuse projecting opposing surfaces. This process is described in U.S. Pat. No. 3,663,680 issued May 16, 1972 to Ringdal.
A hollow-walled structure having spaced apart walls, such as a boat hull or other marine hull, is formed by a rotational molding process. For purposes of structural integrity certain walls of the structure are inter-connected according to the invention. Opposing walls are connected by one or more rows of spaced apart molded connectors or joints.
The molded connectors are formed during the molding process. The mold has two or more sections that are assembled or closed for the manufacturing process. Selected mold surfaces. involved in the formation of the hollow-walled structure are provided with V-shaped indentations that project into the space between one mold surface and an opposing mold surface. The projecting members form a constriction between the two mold surfaces so that molten material fills the space to form a molten joint. The indentations are short and are spaced apart along a row. During molding, molten material flows between the indentations so as not to obstruct the process of molding the entire structure. One or more rows of such molded connecting joints are formed, greatly enhancing the structural integrity of the molded object.
A boat molded by such a process has hollow side walls and a hollow bottom wall formed of an inner bottom wall component and an outer bottom wall component. The bottom wall components are connected by the spaced apart molded joints. The boat can be a bass fishing boat, a duck boat, a pontoon boat, sail boat or a normal utility boat. The inner wall of the boat can be molded according to the requirements of the boat. For example, a bass fishing boat can have a number of compartments, a motor well and a bulkhead for mounting a steering assembly and various instrumentation. Other objects that can be molded include pontoons for a pontoon boat, dock sections and other such hollow walled structures.
In terms of a boat hull, the invention also includes the process of forming trusses in the sidewalls of the boat hull, and a boat hull finished having such interior trusses. The trusses are formed by including a lattice in the sections of the mold that will form the boat side walls. The lattice is connected to the inner mold before the mold is closed. Portions of the latticework are in close proximity to the mold surfaces. When the rotational molding process begins, the thermoplastic material bonds to the lattice, and bonds the lattice to the boat hull walls. The resultant trusses add strength and stability to the side walls of the boat hull. The boat hull can be a conventional boat hull or thermoplastically molded hulls for pontoons in the formation of a pontoon boat.